U.S. patent number 8,508,293 [Application Number 13/405,632] was granted by the patent office on 2013-08-13 for amplitude shift keying demodulator and method for demodulating an ask signal.
This patent grant is currently assigned to Beken Corporation. The grantee listed for this patent is Dawei Guo, Jiazhou Liu, Yangeng Wang. Invention is credited to Dawei Guo, Jiazhou Liu, Yangeng Wang.
United States Patent |
8,508,293 |
Liu , et al. |
August 13, 2013 |
**Please see images for:
( Certificate of Correction ) ** |
Amplitude shift keying demodulator and method for demodulating an
ask signal
Abstract
An ASK demodulator comprises a rectification circuit which
receives and rectifies an ASK signal to generate a rectified
current; an active load circuit is coupled to the rectification
circuit and receives the rectified current and present an impedance
which is inversely proportional to at least a part of the rectified
current when a frequency of a base band signal meets a preset
condition; a comparator is coupled to the rectification circuit and
the active load circuit and receives a reference voltage and a
voltage generated based on, at least in part, the rectified current
and the impedance, and compares the reference voltage and the
generated voltage to generate a demodulated signal.
Inventors: |
Liu; Jiazhou (Shanghai,
CN), Guo; Dawei (Shanghai, CN), Wang;
Yangeng (Shanghai, CN) |
Applicant: |
Name |
City |
State |
Country |
Type |
Liu; Jiazhou
Guo; Dawei
Wang; Yangeng |
Shanghai
Shanghai
Shanghai |
N/A
N/A
N/A |
CN
CN
CN |
|
|
Assignee: |
Beken Corporation (Shanghai,
CN)
|
Family
ID: |
48756237 |
Appl.
No.: |
13/405,632 |
Filed: |
February 27, 2012 |
Prior Publication Data
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|
|
Document
Identifier |
Publication Date |
|
US 20130182797 A1 |
Jul 18, 2013 |
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Foreign Application Priority Data
|
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|
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Jan 16, 2012 [CN] |
|
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2012 1 0012422 |
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Current U.S.
Class: |
329/311; 329/369;
329/347 |
Current CPC
Class: |
H04L
27/06 (20130101); H03D 1/2272 (20130101) |
Current International
Class: |
H03K
9/02 (20060101) |
Field of
Search: |
;329/369,347,311
;375/353 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Chang; Joseph
Attorney, Agent or Firm: Perkins Coie LLP Wininger;
Aaron
Claims
What is claimed is:
1. An ASK demodulator, comprising: a rectification circuit
configured to receive and rectify an ASK signal to generate a
rectified current; an active load circuit coupled to the
rectification circuit and configured to receive the rectified
current and present an impedance which is inversely proportional to
at least a part of the rectified current when a frequency of a base
band signal meets a preset condition; and a comparator coupled to
the rectification circuit and the active load circuit and
configured to receive a reference voltage and a voltage generated
based on, at least in part, the rectified current and the
impedance, and compare the reference voltage and the generated
voltage to generate a demodulated signal; wherein the active load
circuit comprises: a first MOSFET with a source coupled to a power
supply and a drain coupled to the rectification circuit; a
capacitor with a first end coupled to the power supply and a second
end coupled to a gate of the first MOSFET; a second MOSFET with a
source coupled to the gate of the first MOSFET and the second end
of the capacitor, a gate coupled to the drain of the first MOSFET
and a drain which is grounded.
2. The ASK demodulator of claim 1, further comprising a reference
voltage generation circuit coupled to the comparator to provide the
reference voltage which is an average of the generated voltage.
3. The ASK demodulator of claim 2, wherein the reference voltage
generation circuit comprises: a first current source with a first
end coupled to the power supply; a third MOSFET with a source
coupled to a second end of the first current source, the gate of
the first MOSFET, the second end of the capacitor and the source of
the second MOSFET, a gate and a drain coupled to each other
configured to generate the reference voltage; and a second current
source with a first end coupled to the drain and the gate of the
third MOSFET and a second end which is grounded.
4. The ASK demodulator of claim 3, wherein the first current source
is configured to provide a current which is twice a current
provided by the second current source.
5. The ASK demodulator of claim 1, wherein the first MOSFET and the
second MOSFET are P-type MOSFET.
6. The ASK demodulator of claim 1, wherein the impedance is
approximately equal to an inverse of a transconductance of the
first MOSFET when the frequency of the base band signal is within a
first frequency range, and wherein the impedance is inversely
proportional to a direct current component of the rectified current
when the frequency of the base band signal is within a second
frequency range higher than the first frequency range.
7. The ASK demodulator of claim 1, wherein the rectified current
comprises a direct current component and an alternating current
component, wherein the direct current component is determined by,
at least in part, an average power of the ASK signal, and wherein
the alternating current component is determined by, at least in
part, a strength and modulation depth of the ASK signal.
Description
CLAIM OF PRIORITY
This application claims priority to Chinese Application No.
201210012422.1 filed on Jan. 16, 2012, which is incorporated herein
by reference.
TECHNICAL FIELD
The present application relates to amplitude shift keying (ASK)
demodulation, particularly to an ASK demodulator for ASK
demodulation.
BACKGROUND
ASK modulation is widely adopted in communication systems. For
example, in Electronic Toll Collection (ETC) systems, a wake-up
signal broadcasted by an Road Side Unit (RSU) is typically
modulated with ASK.
Conventionally, to conserve power, an ASK signal is demodulated by
means of Schottky Barrier Diodes (SBD) or source followers.
However, SBDs are not compatible with CMOS integration while source
followers can only achieve very limited gains.
Therefore, a new demodulator with controlled power consumption and
adequate gain is required.
SUMMARY OF THE INVENTION
In an embodiment of the invention, an ASK demodulator comprises a
rectification circuit configured to receive and rectify an ASK
signal to generate a rectified current; an active load circuit
coupled to the rectification circuit and configured to receive the
rectified current and present an impedance which is inversely
proportional to at least a part of the rectified current when a
frequency of a base band signal meets a preset condition; a
comparator coupled to the rectification circuit and the active load
circuit and configured to receive a reference voltage and a voltage
generated based on, at least in part, the rectified current and the
impedance, compare the reference voltage and the generated voltage
to generate a demodulated signal.
In an embodiment of the invention, a method for demodulating an ASK
signal comprises receiving and rectifying an ASK signal by a
rectification circuit to generate a rectified current; receiving
the rectified current and presenting an impedance which is
inversely proportional to at least a part of the rectified current
by an active load circuit when a frequency of a base band signal
meets a preset condition; comparing a reference voltage with a
voltage generated based on, at least in part, the rectified current
and the impedance to generate a demodulated signal.
BRIEF DESCRIPTION OF THE DRAWINGS
Non-limiting and non-exhaustive embodiments of the present
invention are described with reference to the following figures,
wherein like reference numerals refer to like parts throughout the
various views unless otherwise specified.
FIG. 1 is a block diagram illustrating a device including an ASK
demodulator according to an embodiment of the invention.
FIG. 2 is a block diagram illustrating a demodulator in FIG. 1
according to an embodiment of the invention.
FIG. 3 is a drawing illustrating an ASK demodulator according to an
embodiment of the invention.
FIG. 4 is a drawing illustrating a relationship between an active
load and a frequency of a base band signal according to an
embodiment of the invention when a bias current is constant.
FIG. 5 is a drawing illustrating a relationship between an active
load and a bias current according to an embodiment of the invention
when a frequency of a base band signal is constant.
FIG. 6 is a drawing illustrating signals at different points in the
ASK demodulator in FIG. 3.
FIG. 7 is a flow chart illustrating a method for demodulating an
ASK signal according to an embodiment of the invention.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
Various aspects and examples of the invention will now be
described. The following description provides specific details for
a thorough understanding and enabling description of these
examples. Those skilled in the art will understand, however, that
the invention may be practiced without many of these details.
Additionally, some well-know structures or functions may not be
shown or described in detail, so as to avoid unnecessarily
obscuring the relevant description. The terminology used in the
description presented below is intended to be interpreted in its
broadest reasonable manner, even though it is being used in
conjunction with a detailed description of certain specific
examples of the invention. Certain terms may even be emphasized
below, however, any terminology intended to be interpreted in any
restricted manner will be overtly and specifically defined as such
in this Detailed Description section.
FIG. 1 is a block diagram illustrating a device 100 including an
ASK demodulator 200 according to an embodiment of the
invention.
As shown in FIG. 1, a device 100 receives an ASK signal which is
demodulated by a demodulator 200 to generate and provide a
downstream circuit 120 a demodulated signal. In various embodiments
of the invention, the device 100 can be any device which receives
an ASK signal. Examples of such devices include, but are not
limited to, on board units (OBU) in an electronic toll collection
(ETC) system. In the ETC system, an ASK signal is generated by a
road side unit (RSU) by modulating a radio frequency carrier (e.g.,
5.83 or 5.84 GHz) by a wakeup signal (e.g., 14 KHz). The
demodulator 200 receives and demodulates the ASK signal so as to
provide the wakeup signal to the downstream circuit 120 (e.g., a
wakeup circuit in this embodiment), wherein the wakeup circuit 120
has been configured to be wakened by a signal with certain
frequency, e.g., 14 KHz.
A low power consumption demodulator with adjustable gain will be
described in detail below with reference to FIGS. 2-5, wherein the
gain may be automatically adjusted in response to the ASK signal,
e.g., an average power of the ASK signal. Therefore, the
demodulator is provided with a dynamic range which is broader
compared with conventional ASK demodulators. For example, when the
average power of the ASK signal increases, in some embodiments of
the invention, the gain decreases so as to prevent the demodulator
from being saturated, i.e., to keep the demodulator capable of
identify digital 1 from digital 0 in the ASK signal. In some
embodiments, when the average power of the ASK signal decreases
(i.e., the ASK signal in this condition is typically referred to as
a "small signal"), the gain will increase so that an additional
amplifier is not required, therefore the demodulator is
particularly adapted to low power circumstances.
FIG. 2 is a block diagram illustrating the demodulator 200 in FIG.
1 according to an embodiment of the invention. The demodulator 200
comprises a rectification circuit 220, an active load circuit 240
and a comparator 260.
When the ASK signal is received by the rectification circuit 220, a
rectified current can be generated because of the non-linearity of
the rectification circuit 220. The rectified current may include a
direct current (DC) component determined by an average power of the
ASK signal and an alternating current (AC) component determined by
a strength and modulation depth of the ASK signal.
The active load circuit 240 is coupled to the rectification circuit
220 and configured to receive the rectified current and present an
impedance which is inversely proportional to at least a part of the
rectified current, e.g., the DC component thereof, when a frequency
of the wake-up signal (base band signal) meets a preset condition.
In an embodiment, the impedance has such a relationship with the
rectified current when the frequency of the base band signal is
within a predetermined frequency range.
A voltage determined by the rectified current and the dynamic
impedance is received and compared with a reference voltage by the
comparator 260. In an embodiment, the reference voltage is equal to
an average of the voltage so that the comparator 260 can identify
digital 1 and digital 0 in the ASK signal. Accordingly, when the
average power of the ASK signal is relatively high, a gain shown by
the generated voltage can be limited by the impedance of the active
load circuit 240, when the average power of the ASK signal is
relatively low, the gain can be enlarged by the impedance of the
active load circuit 240.
The comparator 260 coupled to the rectification circuit 220 and the
active load circuit 240 and configured to receive a reference
voltage and a voltage generated based on, at least in part, the
rectified current and the impedance, compares the reference voltage
and the generated voltage to generate a demodulated signal.
FIG. 3 is a drawing illustrating an ASK demodulator 200a according
to an embodiment of the invention. On the basis of the demodulator
200 shown in FIG. 2, the demodulator 200a further includes a
reference voltage generation circuit 380 configured to generate the
reference voltage.
In this embodiment, the rectification circuit 220 comprises a
capacitor 322, a resistor 324 and a MOSFET 326. One end of the
capacitor 322 configured to received the ASK signal and block any
DC component thereof. One end of the resistor 324 is configured to
receive a bias voltage for the rectification circuit 220. The other
ends of the capacitor 322 and the resistor 324 are coupled to a
gate of the MOSFET 326. A source of the MOSFET 326 is grounded, a
drain of the MOSFET 326 is coupled to the active load circuit 240
and the comparator 260 and is configured to generate the voltage to
be compared with the reference voltage. As described above, a
rectified current (a drain to source current) can be generated
based on the ASK signal by the MOSFET 326 with non-linearity.
The active load circuit comprises, as shown in FIG. 3, a MOSFET 342
(first MOSFET), a MOSFET 346 (second MOSFET) and a capacitor 344.
The impedance presented by the circuit 240 is defined in equation
(1):
.times..times..times. ##EQU00001## Where gm1 is a transconductance
of MOSFET 342, S is a complex frequency of the base band signal
(i.e., j.omega.), C is a capacitance of the capacitor 344, gm2 is a
transconductance of the MOSFET 346, and Ron1 is a drain-source
resistance of MOSFET 342.
Therefore, it can be derived from equation (1) that in DC
condition, S=0 and Ro.about.1/gm1, in case the frequency of the
base band signal is relatively high, R.sub.o=Ron1, by using
first-order approximate solution, the impedance can be defined in
equation (2):
.times..times..lamda..times..times. ##EQU00002## Where .lamda. is a
constant and referred to as channel modulation coefficient,
I.sub.dc is a DC current in the MOSFET 342 (i.e., the DC component
of the rectified current).
From equations (1) and (2), a relationship between the impedance
presented by the active load circuit and the frequency of the base
band signal (e.g., wakeup signal) when a bias current (determined
by average power of the ASK signal) is constant can be determined
as shown in FIG. 4, wherein a corner frequency is determined by gm2
and C.
Referring to equation (2), when the frequency is higher than the
corner frequency, the impedance presented by the active load
circuit 240 may be as shown in FIG. 5. That is, the impedance is
inversely proportional to the DC component of the rectified current
in the MOSFET 326 which is determined by the average power of the
ASK signal. In other words, the higher the DC component is, the
lower the impedance is; the lower the DC component is, the higher
the impedance is. Accordingly, the gain is scaled in response to
the power of the ASK signal, in an embodiment, the gain gets lower
by decreasing the impedance when the ASK signal is stronger, and
the gain gets higher by increasing the impedance when the ASK
signal is weaker (small signal).
Referring back to FIG. 3, the reference voltage generation circuit
380 comprises a current source (first current source) 382, a
current source (second current source) 386 and a MOSFET (third
MOSFET) 384. One end of the current source 382 is coupled to a
power supply, the other end of the current source 382 is coupled to
a source of the MOSFET 384, a gate and a drain of the MOSFET 384 is
coupled to each other and further coupled to one end of the current
source 386, the comparator 260, and are configured to provide the
reference voltage (at N4).
In an embodiment, the reference voltage is an average of the
voltage generated based on the rectified current and the impedance
(i.e., generated at N3). The other end of the current source 386 is
grounded. Specifically, a current provided by the current source
382 is twice a current provided by the current source 386,
therefore a current flowing pass the MOSFET 346 will be the same as
the current flowing pass the MOSFET 384. In absence of the ASK
signal, the voltages at N3 and N4 are the same. When the ASK signal
is received by the demodulator 200a, the voltage at N4 changes with
an average power of the ASK signal and is equal to an average of
the voltage at N3. The comparator 260 compares the voltages at N3
and N4 to generate the modulated signal which can be provided to
the downstream circuit 120 (FIG. 1).
FIG. 6 is a drawing illustrating signals at different points in the
ASK demodulator in FIG. 3.
As shown in FIG. 6, it can be seen that in the demodulator 200a,
even though there is no additional amplifier, the difference
between the voltages at N3 and N4 are large enough for the
comparator 260 to identify digital 1 and digital 0 in a base band
signal (e.g., wakeup signal) appropriately.
A demodulator according to different embodiments of the invention
can operate with a satisfying performance even though an ASK signal
received is relatively strong and has a non-100% modulation depth.
That is, such a demodulator has a broader dynamic range than
conventional ASK demodulators.
FIG. 7 is a flow chart illustrating a method 700 for demodulating
an ASK signal according to an embodiment of the invention. The
method 700 will be described with reference to FIGS. 2, 3 and
7.
At block 720, the rectification circuit 220 (FIG. 3) receives and
rectifies an ASK signal by a rectification circuit to generate a
rectified current.
At block 740, the active load circuit 240 receives the rectified
current and presents an impedance which is inversely proportional
to at least a part of the rectified current by an active load
circuit when a frequency of a base band signal meets a preset
condition;
At block 760, the comparator 260 compares a reference voltage with
a voltage generated based on, at least in part, the rectified
current and the impedance to generate a demodulated signal.
In an embodiment, the reference voltage is an average of the
generated voltage.
In an embodiment, the impedance is approximately equal to an
inverse of a transconductance of the first MOSFET when the
frequency of the base band signal is within a first frequency
range, and wherein the impedance is inversely proportional to a
direct current component of the rectified current when the
frequency of the base band signal is within a second frequency
range higher than a second corner frequency.
In an embodiment, the rectified current comprises a direct current
component and an alternating current component, wherein the direct
current component is determined by, at least in part, an average
power of the ASK signal, and wherein the alternating current
component is determined by, at least in part, a strength and
modulation depth of the ASK signal.
This written description uses examples to disclose the invention,
including the best mode, and also to enable any person skilled in
the art to practice the invention, including making and using any
devices or systems and performing any incorporated methods. The
patentable scope of the invention is defined by the claims, and may
include other examples that occur to those skilled in the art. Such
other examples are intended to be within the scope of the claims if
they have structural elements that do not differ from the literal
language of the claims, or if they include equivalent structural
elements with insubstantial differences from the literal languages
of the claims.
* * * * *